This invention relates to deflection structures for cathode ray tubes. More particularly, this invention relates to a shielded helix traveling wave deflection structure for a cathode ray tube.
Helically shaped coils have been used as slow wave deflection structures in cathode ray tubes (CRTs)for many years. They deflect the electrons in a CRT beam by creating a deflecting field that travels at the same velocity as the electrons in the beam, thereby gaining a high sensitivity and good bandwidth. Several different types and modifications of such structures have been described in the patent literature. For example, Kluver U.S. Pat. No. 3,153,742 describes an electron tube delay device wherein an electron beam travels down a helical drift path; an input cavity resonator which transfers signal wave energy to the beam; and an output cavity resonator which extracts signal wave energy from the beam. The beam is directed down the helical path by a trough-shaped negative electrode that produces an electric focusing field with a surrounding cylindrical electrode.
Correll U.S. Pat. No. 4,812,707 discloses an electron beam deflection structure of the traveling wave type comprising a first helical coil and a second helical coil interleaved with one another and coaxial with the axis of the tube. The first coil has wide segments positioned on the bottom with narrow segments on the top while the second coil has wide segments on the bottom and narrow on the top. Differential voltage signals of opposite polarity are applied to the first and second coils.
Nishino et al. U.S. Pat. No. 3,696,266 shows an electron beam deflecting device with a helical electrode coaxially surrounded by a cylindrical outer electrode. By tapering the helical strip structure forwardly on its side defining the passage of the electron beam, the center line of the beam is deviated for an angle one-half that of the taper.
Odenthal et al. U.S. Pat. No. 3,694,689 and Reissue U.S. Pat. No. 28,223 describe a helical delay line deflection apparatus to reduce the deflection signal in the axial direction along the helical deflector until it is equal to the electron beam velocity to permit very high frequency signals to deflect the beam without appreciable distortion. A pair of helical deflector members having rectangular turns are provided, each having a pair of flat side portions separated by a deflector portion of different width. Two pairs of grounded adjustable compensator plates are positioned adjacent the flat side portions on opposite sides of both helical members to form delay lines. The width and spacing of adjacent deflection portions is substantially uniform while the width and spacing of adjacent side portions varies for successive turns of the path along the electron beam to provide good response deflection sensitivity with an extremely wide band width frequency response.
Crandall U.S. Pat. No. 3,916,255 shows a phase array amplifier for generating multiple channels of high frequency electromotive power. Each channel contains an electron beam gun, to generate an electron beam and deflection plates along the beam path to modulate the beam. In one embodiment, the deflector plates comprise a helical electrode and a control grounded electrode.
Loty et al. U.S. Pat. No. 3,376,464 discloses a cathode ray apparatus which includes a flat helical deflection electrode and a second electrode having a part within the helical electrode disposed along its axis and an outer part practically completely surrounding the helical electrode.
Christie et al. U.S. Pat. No. 4,093,891 describes an electron beam deflection apparatus comprising a pair of diverging flat helically wound coils disposed on opposite sides of the beam axis.
Tomison et al. U.S. Pat. No. 4,207,492 describes an electron beam deflection structure comprising serpentine deflection plate segments which are interconnected together by elongated loops.
Piazza et al. U.S. Pat. No. 3,849,695 shows a deflection structure for a cathode ray tube which comprises a helical electrode bonded to the teeth of a comb-like dielectric support structure. The electron beam passes between the helical electrode and a ground plane. In one embodiment, two such helical electrodes oppose one another and two opposing ground plane electrodes are situated adjacent the helical electrodes with the beam passing down the middle between the opposing helical electrodes and ground plane electrodes.
Chang U.S. Pat. No. 4,429,254 discloses an electron beam deflection yoke comprising four rod-shaped members running parallel to the axis of the beam and disposed around the beam axis, each having a wire coil wound along the rod to form horizontal and vertical deflection coils.
However, in most of these devices a serious problem exists which causes a "precursor" artifact to appear on the CRT trace in advance of the rise time when a fast rising pulse is measured by the CRT. This spurious signal is caused by fields which are not confined to the helical transmission line or deflection coil, but are coupled between turns of the helical coil or coils, or are transmitted by higher velocity modes in the space between the helix and the ground planes. These fast fields deflect the electron beam at an earlier time than the correct time in the pulse and cause an erroneous signal to appear where there should be none. This artifact or spurious signal is more serious and detrimental in high bandwidth CRTs, i.e., CRTs having a bandwidth of about 2 GHz (2000 MHz) and above, because the high frequency components of these signals are preferentially coupled along the structure.
It would, therefore, be desirable to provide a helical coil type deflection structure for a cathode ray tube device which would eliminate the propagation of such fast signals along the helical deflection structure.